1. Field of the Invention
The present invention generally relates to a direct fuel injection engine in which a fuel is directly injected in a combustion chamber and ignited by a spark plug. More specifically, the present invention relates to a direct fuel injection engine that has a piston with a pair of cavities that are arranged to receive the injected fuel at an angle relative to the piston axis for obtaining an excellent stratified fuel-air mixture.
2. Background Information
Fuel consumption of an internal combustion engine can be greatly reduced, particularly in low to medium load regions, when the internal combustion engine is configured to inject fuel directly into the combustion chamber from a fuel injection valve and very lean combustion is conducted by forming a stratified fuel-air mixture inside the cylinder. In order for this kind of direction fuel injection engine to ignite and combust the fuel-air mixture in a stable and reliable manner, it is important to form a well-stratified fuel-air mixture agglomeration inside the cylinder such that the size and air-fuel ratio of the fuel-air mixture agglomeration is appropriate for the rotational speed and load of the engine.
With this kind of direction fuel injection engine, one known method of forming an optimum fuel-air mixture over a wide range of operating conditions is to provide a fuel injection valve oriented generally parallel to the center reciprocation axis of the piston and arranged near the center of an upper area of the combustion chamber and injecting fuel toward a cavity provided in the top surface of the piston so as to form a stratified fuel-air mixture. An example of a direct fuel injection internal combustion engine that employs a center injection arrangement in which a fuel injection valve and spark plug are arranged on or near the reciprocation axis of the piston is disclosed in Japanese Laid-Open Patent Publication No. 2000-265841.
In this publication, the fuel injection valve and spark plug are arranged on or near an upper area of the combustion chamber that is adjacent the center reciprocation axis of the piston. In this publication, the shape of a fuel streams injected from the fuel injection valve form a hollow cone that strikes the top surface of the piston. The top surface of the piston is provided with a cavity comprising a central deep dish or cavity and a plurality of shallow dishes or cavities that are arranged around the perimeter of the deep dish or cavity. By varying the injection timing in accordance with the operating conditions, the forming state of the stratified fuel-air mixture can be changed and stratified operation can be accomplished over a wide range of operating conditions. More specifically, the fuel stream can also be injected to either the deep cavity or the shallow cavities by varying the fuel injection timing, thereby enabling the stratified state of the fuel stream to be changed (i.e., switched between stratified combustion and weakly stratified combustion). When operating in a region of low to medium speed, the fuel injection timing is controlled to occur during a latter half of the compression stroke such that the fuel stream hits the deep cavity. When the fuel stream hits the deep cavity, a high stratified state is obtained in which an agglomerate fuel-air mixture forms inside the deep cavity and there above. When operating in a region of high speed, the fuel injection timing is controlled to occur during a former half of the compression stroke such that the fuel stream hits the shallow cavities. When the fuel stream hits the shallow cavities, the fuel stream is guided by lateral wall surfaces of the shallow cavities such that it swirls upward. As a result, a low stratified state is obtained in which a fuel-air mixture agglomeration is formed inside the shallow cavities and there above. Thus, combustion can be accomplished with good fuel efficiency.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved direct fuel injection engine. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.